This invention relates to a semiconductor device, and more particularly to a semiconductor device of the plastic mold type using a bedless lead frame.
Because plastic mold type semiconductor devices are easy to mass-produce and can be manufactured at a low cost, they have been widely used in recent years. Such semiconductor devices have been used also in large semiconductor devices having far more than 100 pins.
In conventional plastic mold type semiconductor devices, a lead frame having a bed portion for mounting a pellet is ordinarily used. However, the problems which occur in using the conventional lead frame are as follows: First tie bars for fixing the bed portion to the outer frame hinder the arrangement of the inner leads, resulting in a lowered area efficiency. Second is that the tie bars serve as a route for moisture to penetrate from the outside. Third is that a crack due to the difference between the coefficient of thermal expansion of the bed portion and that of the plastic resin for mold is likely to occur. Fourth is the troublesome design and the poor economic efficiency because the dimensions of the bed portion must be changed in conformity with the dimensions of the pellet used.
To overcome such problems, a bedless semiconductor device has been proposed. FIG. 1 shows a cross section of the bedless semiconductor device, wherein leads 15 extend closer to the vicinity of the center of the semiconductor device, when compared to those of the conventional one, and no bed portion is provided. An insulating sheet (tape) 16 is fixed onto substantially the central portion of the semiconductor device in a manner where it is supported by the leads 15. After a pellet 12 is subjected to die-bonding so that it is mounted on the insulating sheet 16, wire-bonding is implemented to connect the pellet 12 and the leads 15 by means of wires 13.
FIGS. 2A, 2B and 2C show how mounting using such bedless type lead frames are conducted. In FIG. 2B, a small pellet 12 is mounted on the central portion of the lead frame 20 shown in FIG. 2A through a small insulating sheet 16. On the other hand, in FIG. 2C, a pellet 12' larger than that in the case of FIG. 2B, is mounted on the same portion as the above through a large insulating sheet 16'.
FIG. 3 is a plan view in which the central portion of the bedless type lead frame shown in FIG. 2A is enlarged. A configuration is provided such that tip portions of respective inner leads 15a are substantially equidistant from the center of the lead frame.
A similar bedless type lead frame is also described in the Nikkei Microdevices, Dec. 1, 1987, Nikkei McGrow-Hill, pp. 76-78 and the Nikkei Electronics, Nov. 16, 1987, Nikkei McGrow-Hill, pp. 100-101.
When such a bedless type lead frame is employed, mounting of pellets of different sizes is enabled by using the same kind of lead frames. Thus, productive management is facilitated and costs can be reduced.
However, because a large number of inner leads are all directed toward the center of the lead frame as shown in FIG. 3 in the conventional bedless type lead frame, the tip portion of each inner lead becomes extremely thin, so that the ability to support the insulating sheet at the central portions of the lead frame is extremely reduced, leading to the problem of stable supporting becoming impossible.